Medical Aspects of Diving: Effects of Elevated Partial Pressures of Gases

[Music] [Music] e anytime a diver goes underwater for an extended period he must utilize some means for supplying himself with an atmosphere containing a suitable amount of oxygen furthermore this atmosphere must be provided at a pressure that always balances the pressure exerted by the surrounding water the deeper the diver goes the greater the water pressure becomes and correspondingly the greater must be the pressure of the atmosphere the diver breathes this is most essential if the diver is to avoid injury from the mechanical effects of pressure but breathing air or any other suitable atmosphere at pressures greater than normal namely one atmosphere absolute exposes the diver to several other potential hazards for example Narcosis a kind of intoxication that leaves the diver unable to recognize or cope with Danger convulsions a violent loss of muscular control unconsciousness a condition that soon ends in drowning if help is not immediately available and If this happens ascending too rapidly as in a blow up it can lead to life-threatening air embolism or decompression sickness a condition whose symptoms range from Pain to disorders incompatible with life to help you avoid these hazards this film will explain how your body is affected by what you breathe and the pressure at which you breathe it the air we normally breathe exerts an absolute pressure of one atmosphere at this pressure air supports combustion readily that is to say it contains oxygen sufficient in amount not only for burning but for sustenance of life as well how much oxygen is in the air can be approximated by this simple experiment as the burning candle uses up the oxygen within the inverted container the water rises to take its place when the flame goes out the water level reaches the 4 fths Mark proof that about 1/5 of the air is oxygen the remaining four fifths of the Air's volume is nitrogen it's Absol solute pressure is now one atmosphere because the rising water compressed the nitrogen until its pressure equal that of the outside air but when this volume of nitrogen fills the entire container as it did at the start of the experiment its pressure is only 8/10 of an atmosphere since the nitrogen in air exerts aess pressure of 8/10 of an atmosphere and air a pressure of one atmosphere the difference 2/10 of an atmosphere is the pressure exerted by oxygen these values are called partial pressures these are the symbols for nitrogen and oxygen the gases in air exert pressure because they are composed of billions upon billions of infinitesimally small particles called molecules which are always in motion each time a molecule strikes the container it exerts a force or pressure against it the pressure we read on this gauge is a measure of the total force exerted by all the molecules that strike the container at any instant 1/5 of these molecules are oxygen molecules therefore the pressure they exert is 1/5 of the total pressure or 1/5 * 1 atmosphere namely 2/10 of an atmosphere since 4 fifths of the molecules are nitrogen molecules the pressure they exert is 4 fths of the total pressure in this instance 4 fths time one atmosphere that is 8/10 of an atmosphere from this this you can see that the partial pressure of each gas in air is obtained by taking the part each gas is of the total volume and multiplying it by the total pressure now let's compress the air volume to 1/2 so the total pressure becomes two atmospheres as you would expect from Bo's law since 1/5 of this reduced volume is still oxygen and four fths of it nitrogen their partial pressures now become 4/10 of an atmosphere and 1 and 610 atmospheres changing the total pressure of air or any other mixture of gases changes the partial pressure of each gas in the mixture but here's another way way to change the partial pressure of a gas the number of oxygen molecules is now only 1/10th of the total number or in other words its volume is 1/10th of the total volume therefore the partial pressure of oxygen now becomes 1/10th times the total pressure here one atmosphere for a value of 1/10th of an atmosphere breathing oxygen at a partial pressure less than normal that is less than 2/10 of an atmosphere produces certain physiological reactions this individual has been breathing air which has been slowly depleted of its oxygen content the oxygen level is now about 10% in other words about 1/10th of the air being breathed is oxygen the individual has a feeling of well-being which keeps him from realizing the danger he's in his metal and motor impairment make him act almost as if he were intoxicated soon he loses Consciousness and collapses to understand better why these physiological disturbances occur let's consider first the effect the partial pressure of oxygen has upon its solubility in blood whenever a gas is in contact with a liquid some of its molecules enter and distribute themselves throughout the liquid until a condition is reached where as many molecules enter the liquid as leave it when this balance exists the partial pressure of the gas above the liquid is equal to the partial pressure of the gas dissolved in the liquid now if the partial pressure of the gas above the liquid is increased by doubling the number of its molecules more gas molecules dissolve into the liquid than leave it this continues until the partial pressure of the gas within the liquid again equals that of the gas above it this type of gas exchange takes place all the time within our bodies the air we breathed when it reaches the very small air sacks which make up the lungs contains oxygen which normally has a partial pressure much greater than the partial pressure of oxygen in the blood carried to the lungs therefore oxygen molecules pass from the air sacs into the blood until the partial pressure of oxygen in the blood when it leaves the lungs equals that in the air saxs the arterial blood carried to the tissues such as the brain has a partial pressure of oxygen greater than that within the tissues as a result oxygen passes from the blood into the tissues but when the atmosphere's partial pressure of oxygen becomes increasingly less than normal the arterial blood's partial pressure of oxygen also becomes increasingly less this causes the oxygen partial pressure between the blood and tissues to become less and less with the result that less and less oxygen enters the tissues the tissues soon become unable to function properly and those within the brain die within a few minutes this will happen to this individual if the air he is breathing is not restored to its normal content of oxygen under normal conditions the air you inhale has an oxygen content of about 20% Which is equivalent to a partial pressure of 2/10 of an atmosphere the air you exhale has an oxygen content of 16% which means that anytime you inhale air with an oxygen content less than 16% that is to say a partial pressure of 1600s or less you are in serious trouble which if not corrected quickly will cause death now let's observe the effect of breathing air in which the partial pressure of oxygen is considerably higher than normal the individual on whom this demonstration is being performed has agreed to this exposure he is restrain from excessive movement in order to keep him from injuring himself after 15 minutes of breathing oxygen at a partial pressure of four atmospheres absolute the individual manifests involuntary localized muscular twitching such is in the eyes and hands this soon becomes generalized finally the victim becomes convulsive a condition which if it occurs underwater is almost certain to end fatally from tests such as this and others conducted by the experimental diving unit it has been established that the average diver during working Dives should not breathe oxygen at a partial pressure of two atmospheres for more than 30 minutes since individuals vary greatly in their susceptibility to oxygen poisoning all diving candidates must demonstrate their ability to breathe pure oxygen within the established safe time depth limits any unusual symptoms such as nausea anxiety restlessness dizziness or muscular twitching is a Telltale indication of oxygen poisoning and calls for an immediate reduction in the partial pressure of oxygen you will recall that as you descend the pressure of your Air Supply must increase by one atmosphere for each 33 ft of descent this is essential if you are to avoid the harmful mechanical effects of pressure thus the absolute pressure at 33 ft is two atmospheres at 66 ft three atmospheres and so on the partial pressures of oxygen at these depths increase correspondingly by 2/10 of an atmosphere since 1/5 of the total pressure is due to the oxygen molecules the partial pressure of oxygen established as the safety limit for Navy divers breathing air is reached at 297 ft but well before this depth is reached the diver is subjected to another danger intoxication or Narcosis by nitrogen at high partial pressure in the air being breathed the effects of breathing nitrogen at a high partial pressure could be demonstrated more clearly in this recompression chamber this man is breathing air at an absolute pressure of 10 atmospheres attained at a depth of 297 ft notice how erratic and unsure his actions are he appears sleepy and not too interested or concerned with the problem at hand this man on the other hand also at 10 atmospheres absolute is breathing a helium OCT texture substituting helium for nitrogen in the breathing medium minimizes or eliminates the narcotic reactions noted when air is breathed at pressures above four atmospheres to avoid the hazards of nitrogen Narcosis special equipment has been designed to supply a helium oxygen breathing medium to divers descending to depths where the absolute pressure is considerably greater than seven atmospheres but regardless of whether the diver breathes an oxygen helium mixture or compressed air the nitrogen or helium in the breathing medium can cause trouble in another way when you breathe air under normal conditions that is at a pressure of one atmosphere the nitrogen that enters your lungs is at a partial pressure of 8/10 of an atmosphere within the lungs the nitrogen in the air Sachs is at the same partial pressure as the nitrogen in the blood entering the lungs as a result the blood leaving the lungs receives no additional nitrogen when this Blood reaches the tissues such as the brain its partial pressure of nitrogen also equals that in the tissues consequently no additional nitrogen enters the tissues but if the diver breathes Air at a pressure greater than one atmosphere the partial pressure of nitrogen in the air Sachs increases this causes the blood leaving the lungs to contain more nitrogen which eventually causes the tissues to contain more nitrogen with each increase of air pressure a considerable period of time is required for the blood to raise the partial pressure of nitrogen in all the tissues to that in the air sacs now if the air being breathed decreases in pressure the process is reversed nitrogen passes from the blood into the air saxs and is hailed this blood in turn receives more nitrogen from the tissues and carries it to the air Sachs where it is released and exhaled this process continues until the partial pressure of nitrogen in all the tissues equals that of the air saxs the rate of ascent determines the rate at which the nitrogen dissolved in the tissues and blood is released this can be demonstrated by a simple experiment the liquid flowing over the glass beads has been exposed to nitrogen Under Pressure until the pressure of its dissolved nitrogen equals the pressure of the surrounding atmosphere of nitrogen decreasing the pressure of the surrounding atmosphere of nitrogen slowly that is to say a slow rate of asent the nitrogen dissolved in the liquid enters the surrounding atmosphere without any visible evidence of its release but when the rate of asent is fast notice the bubbles of nitrogen that form in the liquid before escaping to the atmosphere here is further evidence of the formation of these n nitrogen bubbles in the blood of a frog which has been rapidly decompressed notice how these bubbles in the blood vessel can block the flow of blood the formation of such bubbles of nitrogen in any of the body's tissues or blood is known as decompression sickness these bubbles interfere with the blood circulation as well as press upon nerves decompression sickness occurs whenever the ascent is too rapid the symptoms of decompression sickness May range from pain in the arms and legs to collapse with unconsciousness decompression sickness is a most serious condition that requires Immediate Care to prevent permanent damage and perhaps death another source of difficulty to the diver is the accumulation within the tissues of carbon dioxide a natural product of body metabolism by breathing the body rids itself of its excess carbon dioxide under normal conditions of rest the air we exhale contains about 4% of carbon dioxide here represented by the Peaks on the right the values on the left represent the percent of carbon dioxide in the air being inhaled which under normal conditions is negligible in amount and of no real concern to the diver but if the exhaled air with its excess of carbon dioxide should become a part of the inhaled air the percent of carbon dioxide in the inhaled air rises causing more carbon dioxide to accumulate within the tissues the body reacts by breathing faster and deeper in order to rid itself of the extra carbon dioxide this is a visual record of the subject's respiratory movements first breathing normal air in which the carbon dioxide content is negligible and then air in which the carbon dioxide content is increasing when the inhaled air contains about 6% of carbon dioxide the breathing becomes noticeably labored the subject is really in distress as his his body hungers for more and more good air with which to flush out the accumulated carbon dioxide the Toxic effect of too much carbon dioxide is eventually unconsciousness unconsciousness usually results when the inhaled air contains 10% or more of carbon dioxide 10% of carbon dioxide at the surface or at one atmosphere absolute has a partial pressure of 1/10th of an atmosphere this value which produces unconsciousness can also be obtained from breathing air with only 1% of carbon dioxide at an absolute pressure of 10 atmospheres therefore to avoid carbon dioxide poisoning when diving with the Navy stand standard dress the diver as he descends must continually increase the rate of flow of air through his suit so that the percent of carbon dioxide in his inhaled air will decrease as the absolute pressure increases and in using closed circuit scuba gear in which the diver's exhaled air becomes part of his inhaled air it is imperative that the equipment remove all the carbon dioxide from the exhaled air the carbon dioxide absorption canister must be watertight and its absorbent crystals unexhausted they should be dry and pink or white never blue in review it is important for every diver to understand how his body is affected by what he breathes and the pressure at which he breathes it for example breathing oxygen at a partial pressure considerably less than 2/10 of an atmosphere results in metal and muscular impairment and finally collapse and unconsciousness while on the other hand breathing oxygen at a partial pressure considerably greater than normal leads to convulsions a condition marked by violent loss of muscular control breathing air at pressur is greater than four atmospheres exposes the diver to nitrogen Narcosis a condition that leaves the the diver unable to work safely or to think effectively divers exposed to Great pressures for considerable periods of time must remember to ascend slowly otherwise they run the risk of falling victim to decompression sickness a condition in which bubbles of nitrogen or helium form within the tissues and blood to produce symptoms which range from Pain to C lapse and unconsciousness divers are also subject to carbon dioxide poisoning if their breathing medium contains carbon dioxide in amounts appreciably greater than normal with carbon dioxide poisoning breathing becomes labored and if severe will cause unconsciousness diving whether it be with scuba or the Navy standard dress is safe when you not only know the Navy's rules for safe diving but even more important when you understand why they have been established by following these rules intelligently every diver can avoid the harmful effects of pressure both mechanical and physiological the